This invention relates to a diesel engine wherein an injection nozzle tip is offset from the center of an offset quadrangular cavity formed in the crown of a piston.
In U.S. Pat. No. 3,872,841, assigned to the assignee of the present invention, the power output and the exhaust emission of a diesel engine are improved by obliquely directing fuel jets to the respective inside walls of a quadrangular cavity formed in the piston crown, and the engine according to this patent has been recognized as yielding very satisfactory performance characteristics. In the course of such development it has been found that the fuel impingement points at the respective side walls of the cavity, the fuel impingement angles, and the fuel travel distances from the nozzle orifices to the cavity walls are extremely important factors in improving engine output and exhaust emissions.
In the combustion chamber of the above patent the central axis of the quadrangular cavity V", as shown by a solid line in FIG. 1, coincides with the center of the piston P, and the fuel injection nozzle N is positioned at the center C of the piston. The fuel jets J.sub.1 " to J.sub.4 " simultaneously and symmetrically impinge on the cavity walls at predetermined points Z.sub.1 " to Z.sub.4 ". Thus, the areas between adjacent fuel jets are uniform and each of the jets is uniformly diffused in the cavity with the same amount of air between adjacent jets, whereby uniform air-fuel mixture and efficient combustion is obtained.
In a small, high speed engine having a high output, however, the intake valve K and exhaust valve H must be enlarged to obtain high intake and exhaust efficiency, whereby the cavity V" must be offset from the center of the piston in view of air turbulence effects, as shown by the dotted lines in FIG. 1. If the intake and exhaust valves are enlarged, however, it becomes impossible to position the injection nozzle at the center of the piston. The nozzle N must therefore be offset from the center C.sub.1 of the offset cavity to point C.sub.2 in order to obtain enough space to mount the nozzle. That is, if the intake and exhaust valves are enlarged the remaining space in the cylinder head is reduced, and the position of the injection nozzle must therefore be changed from C.sub.1 to C.sub.2.
Such nozzle disposition results in unequal fuel impingement lengths and different air spaces between adjacent jets, however, whereby the fuel distribution and uniformity of the air-fuel mixture is degraged, resulting in poorer combustion characteristics. More specifically, the positions of C.sub.1 and C.sub.2 are determined in accordance with various engine operating conditions, and the fuel impingement points Z.sub.1 ' to Z.sub.4 ' are thus fixed. The distances between the imaginary points a to d defined by the corner intersections of the cavity side wall extensions and the points Z.sub.1 ' to Z.sub.4 ' are made equal to each other, and the four injection nozzle orifices are directed to impinge the fuel jets J.sub.1 ' to J.sub.4 ' at points Z.sub.1 ' to Z.sub.4 '. With this arrangement the maximum length fuel jet J.sub.1 ' is much longer than the minimum length fuel jet J.sub.3 ', however, which degrades the air-fuel mixture characteristics.